An Al-Qaida-linked insurgent group posted a Web video Friday showing what it said was the downing of a US military helicopter earlier this week. Seven Americans were killed in the crash. The US Marines commandant Gen. James Conway has said the Marines do not believe that the CH-46 Sea Knight helicopter was shot down in the crash Wednesday northwest of Baghdad. US military officials said early indications were that the craft's loss was due to mechanical failure. The two-minute video, which says it shows the "downing of US aircraft on Feb. 7," shows a helicopter that appears to be a Sea Knight flying. At least one object trailing smoke is seen in the sky nearby, and then the craft bursts into orange and red flames, with a spray of debris emerging from it. It is not clear whether the object is a rocket, and it cannot be clearly seen connecting with the craft.